Question: Can Your Hair Turn White from Fright or Turn Gray Overnight?
Answer: You've heard tales of extreme fright or stress turning a person's hair suddenly gray or white overnight, but can it really happen? History records that the hair of some condemned prisoners [e.g., Thomas More (1535) and Marie Antoinette (1793)] turned white overnight before their executions. However, I am unaware of any modern reports of this happening. It's documented your apparent natural haircolor can change over the course of several weeks/months from conditions that affect your hormones (such as pregnancy) or from taking certain medications (like chemotherapy), but can you go gray overnight? Let's look at the chemistry of hair to answer the question.
Hair is a protein that gets its natural color from the presence of a pigment called melanin. Anyone who has bleached their hair can tell you it's chemically possible to render the melanin colorless. Therefore if your sudden fright has something to do with exposure to ionizing radiation or bleach, I can see white hair being a possible outcome, though less likely than baldness or death. Seriously, unless you are playing with toxic radiation or chemicals, you can't instantly change your hair color.
Can fear or stress or any extraordinary emotion change the color of your hair? Yes, but not instantly. Your psychological state has a significant impact on the hormones that can effect the amount of melanin deposited in each strand of hair, but the effect of emotion takes a long time to see. The hair you see on your head emerged from its follicle a long time ago. So, graying or any other color change is a gradual process, occurring over the course of several weeks, months, or years.
Your emotions can't instantly change the color of your hair, but it is possible you could turn gray overnight. How? A medical condition termed "diffuse alopecia areata" can result in sudden hair loss. The biochemistry of alopecia isn't well understood, but in people who have a mix of dark and gray or white hair, the uncolored hair is less likely to fall out. The result? A person can appear to go gray overnight. Although I didn't find any references on this, the implication to me is your hair thins or you become bald if you don't have any gray hair, which for some reason is less resistant to the effect. Sudden hair loss can be caused by certain drugs, medical conditions, or by sudden stress. It tends to occur over the course of several days/weeks, but the result is still dramatic.